In some commonly known electromagnetic clutches having a stationary magnetic core, a rotor and a relatively rotatable armature, an air gap separates the rotor from the armature when the electromagnet is de-energized. The armature is held away from the rotor by means of leaf springs secured to a pulley assembly which in turn is keyed to the shaft about which the clutch rotates. A multiple-turn winding (i.e., clutch coil) is carried by the magnetic core and, when energized, produces magnetic flux which threads a path through the magnetic core, the rotor and the air gap to the armature whereby the armature is drawn toward the rotor. By way of this flux coupling, the armature is moved to close the gap and engage the rotor so the two are coupled by friction and one drives the other without slippage. The coupling torque between the rotor and the armature is dependent in part upon the m.m.f. produced by the coil and the magnetic force created by flux threading the interface between the rotor and the armature.
Typically, when full or rated voltage is applied to an initially de-energized clutch coil (i.e., a step voltage), the current rises exponentially due to the inductance of the coil. In a gap-type electromagnetic clutch, at a predetermined level of current the m.m.f. in the magnetic path becomes sufficient to pull the armature into contact with the rotor against the bias of the springs. At the instant of gap closure (touching of armature to rotor) the coil current and the m.m.f. may have almost reached the rated or maximum values, but the flux is still rising because the reluctance of the entire flux path falls dramatically as the gap narrows and closes. Because torque transmission between a touching rotor and armature is generally proportional to the flux crossing the interface, if rated voltage is applied at a first instant to the coil, the armature more or less slams into engagement with the rotor at a later second instant with a slight delay determined by coil inductance and mechanical inertia. But at the second instant, torque transmission between the rotor and armature virtually jumps from zero to the rated value.
Such jump in torque may cause (i) an undesirably sudden loss in speed of the prime mover supplying input power to the clutch, (ii) undue shock or strain on driving or driven components, including belts or chains, and (iii) unpleasant engagement noise and belt screech. In addition, when the rotor and armature are engaged, the inertia of the slower moving of the two (and its load) needs to be overcome before the full torque coupling locks the rotor and armature into synchronized rotation. After touching (i.e., initially after gap closure) and while the load inertia is being overcome, frictional slippage occurs at the rotor-armature interface; but due to existence of the maximum or rated magnetic attraction force, this slippage is not smooth; instead, it involves stick-slip action (alternate slips and holds) which produces chatter noise and undue wear at the interface. This alternating slip-hold vibratory engagement or chatter is sometimes evidenced by a loud audible vibration or "screeching" noise generated at the rotor-armature interface.
Some clutches and brakes have been associated with control units which produce a so-called "soft start" action. In these, the average coil current and the average m.m.f. are smoothly increased from zero to maximum or rated values. This works satisfactorily for clutches and brakes in which the armature and rotor are not separated by a gap, but instead relatively rub with light contact when the clutch is "disengaged". In this sort of arrangement, slippage gradually decreases, torque gradually increases and "chatter" does not occur. Mechanical shocks on a prime mover and associated driven components are alleviated when a gapless type clutch or brake is excited with a smooth ramp to produce a "soft start".
Applicant has discovered that when a gap-type clutch is brought into engagement with a so-called "soft-start" control unit, the armature is not shifted--against the force of the biasing springs to cross the gap and touch the rotor--until the gradually rising average current and average m.m.f. have reached, or almost reached, their rated or maximum values. Thus, by the instant that the cooperative friction faces come into contact, the magnetic force of attraction is essentially at its maximum, and a sudden, large step change in torque is experienced with all the noise, shock and wear problems described above. Ramping the average voltage and current works well for zero-gap magnetic clutches, but it will not solve the problems for a gap-type clutch or brake.